Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumened. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide in the presence of heterogeneous or homogenous catalysts into equimolar amounts of phenol and acetone, a co-product.
It is known that phenol and cyclohexanone can be co-produced by oxidizing cyclohexylbenzene to cyclohexylbenzene hydroperoxide and decomposing the cyclohexylbenzene hydroperoxide in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
There are, however, a number of problems associated with producing phenol via cyclohexylbenzene rather than the cumene-based Hock process. Firstly, oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide is much more difficult than oxidation of cumene and requires elevated temperatures and the use of a catalyst, generally a cyclic imide, such as N-hydroxyphthalimide (NHPI), to achieve acceptable rates of conversion. However, cyclic imide catalysts are expensive and, when used to catalyze the oxidation of cyclohexylbenzene, the selectivity to cyclohexylbenzene hydroperoxide decreases with increasing conversion.
According to the invention, it has now been found that the addition of a controlled amount of a solvent/diluent (e.g., benzonitrile) improves the conversion rate dramatically, even despite the reduced concentration of cyclohexylbenzene in the mixture, while maintaining or increasing the selectivity to cyclohexylbenzene hydroperoxide. In addition, since both NHPI and oxygen have higher solubility in benzonitrile than in cyclohexylbenzene, the addition of benzonitrile allows an increased concentration of oxygen and NHPI in the liquid phase, thereby further improving the conversion rate and the hydroperoxide selectivity.
U.S. Pat. No. 7,038,089 discloses a process for preparing a hydroperoxide from a hydrocarbon selected from a group consisting of primary hydrocarbons, secondary hydrocarbons, and mixtures thereof corresponding to said hydroperoxide which comprises conducting oxidation of said hydrocarbon at a temperature in the range between 130° C. and 160° C. with an oxygen-containing gas in a reaction mixture containing said hydrocarbon and a catalyst comprising a cyclic imide compound and an alkali metal compound. Suitable hydrocarbons are said to include C4 to C20 tertiary alkanes (e.g., iso-butane, iso-pentane, iso-hexane, and the like), C7 to C20 (alkyl) aromatic hydrocarbons with 1 to 6 aromatic rings or C9 to C20 (cycloalkyl) aromatic hydrocarbons with 1 to 6 aromatic rings (e.g., xylene, cumene, cymene, ethylbenzene, diisopropylbenzene, cyclohexylbenzene, tetrahydronaphthalene (tetraline), indane, etc.), and the like. The reaction may be carried out in the presence of a solvent, such as nitriles (e.g., benzonitrile, acetonitrile, etc.), organic acids (e.g., formic acid, acetic acid, etc.), nitro compounds (e.g., nitromethane, nitrobenzene, etc.), chlorohydrocarbons (e.g., chlorobenzene, 1,2-dichloroethane, etc.), and mixtures thereof.